1. Field of the Invention
The present invention relates to a method for radio transmission of data between a fixed station and at least one mobile station at one of a plurality of carrier frequencies in time slots using a time division multiplex method and time division duplex method where the change from one carrier frequency to another for a subsequent time slot requires a predetermined time period in the order of magnitude of one time slot, and the data is transmitted in two successive active time slots followed by an inactive time slot in which no data are transmitted. The present invention is also directed to an arrangement for radio transmission of data having a fixed station and at least one mobile station where data is transmitted in time slots using the time-division multiplex method and time-division duplex method as well as at one of a plurality of carrier frequencies using the frequency-division multiplex method.
2. Description of the Related Art
The DECT (Digital Enchanced Cordless Telecommunication) Standard was adopted at the start of the 1990""s in order to replace the various existing analog and digital Standards in Europe. This is the first common European Standard for cordless telecommunications. A DECT network is a microcellular, digital mobile radio network for high subscriber densities and, is primarily designed for use in buildings. However, it is also possible to use the DECT Standard outdoors. The capacity of the DECT network of around 10,000 subscribers per square kilometer provides, from the cordless standard, ideal access technology for network operators. According to the DECT Standard, it is possible to transmit both voice and data signals. Thus, cordless data networks can also be built on a DECT base.
The DECT Standard is explained in more detail in the following text with reference to FIG. 2. A digital, cordless telecommunications system for ranges of less than 300 m has been standardized for Europe under the designation DECT. In conjunction with the switching function of a telecommunications installation, this system is therefore suitable for mobile telephone and data traffic in an office building or on a commercial site. The DECT functions supplement a telecommunications installation, and thus make it the fixed station FS of the cordless telecommunications system. Digital radio links between the fixed station FS and a maximum of 120 mobile stations MS can be produced, monitored and controlled on up to 120 channels.
A maximum of ten different carrier frequencies (carriers) are used for transmission in the frequency range from 1.88 GHz to 1.9 GHz. This frequency-division multiplex method is called FDMA (Frequency Division Multiple Access).
Twelve channels are transmitted successively in time on each of the ten carrier frequencies using the time-division multiplex method TDMA (Time Division Multiple Access). Cordless telecommunication in accordance with the DECT Standard using ten carrier frequencies with twelve channels per carrier frequency provides a total of 120 channels. Since one channel is required, for example, for each voice link, there are 120 links to the maximum of 120 mobile stations MS. The duplex method (TTD) is used on the carriers. After the twelve channels (channels 1-12) have been transmitted, the systems switch to receive, and the twelve channels (1-24) in the opposite direction are received.
A time-division multiplex frame thus comprises 24 channels (see FIG. 2). Channel 1 to channel 12 are transmitted from the fixed station FS to the mobile stations MS, while channel 13 to channel 24 are transmitted in the opposite direction, from the mobile stations MS to the fixed station FS. The frame duration is 10 ms. The duration of a channel (time slot) is 417 xcexcs. 320 bits of information (for example voice) and 100 bits of control data (synchronization, signaling and error check) are transmitted in this time. The useful bit rate for a subscriber (channel) of 32 Kbits/s results from the 320 bits of information within 10 ms.
Integrated modules have been developed to carry out the DECT functions for fixed and mobile stations where the fixed station and the mobile station carry out similar functions. One of these in this case integrated modules is in this case the RF module, i.e. the module which carries out the actual function of receiving and transmitting the RF band.
It is known for fast hopping RF modules to be used, i.e. RF modules which can carry out a change in carrier frequency from one time slot or channel to the next. These fast hopping RF modules are intrinsically very complex and costly. Thus, in practice, slow hopping RF modules are mainly used, modules which require a certain amount of time to change the carrier frequency. In practice, the time period which the slow hopping RF module requires to change the carrier frequency corresponds essentially to the time period of a time slot. This means that, after each active time slot, (i.e., after each slot in which data are transmitted,) an inactive time slot (blind slot) must follow in which no data can be transmitted. Thus, in practice, only six links are available on one carrier frequency to the DECT standard instead of the twelve possible links.
A DECT channel is defined by its time slot and its carrier frequency. In this case, it should be noted that the organization to reuse physical channels in accordance with the DECT Standard is carried out by way of dynamic channel selection, so there is no need for any complex frequency planning, as in cellular systems. To set up a link, the signal levels of all the channels are measured continuously, and the interference-free channels are controlled in a channel list (channel map). While a link exists, the signal levels of all the channels and the reception quality continue to be monitored. If this monitoring indicates that the channel currently being used has been transmitted at a carrier frequency which is subject to interference (for example, as a result of the influence of a transmission at the same carrier frequency from or to another fixed station), another carrier frequency is automatically selected for the next active time slot and is entered in the channel list as being interference-free. This represents one option for the organization of the reuse of the channels.
Alternatively, for example, a so-called frequency hopping method can also be used, in which the carrier frequency is changed after a predetermined time period, for example, a transmission frame.
For nationals outside Europe, the DECT Standard may need to be modified and matched to local conditions. For example, in the USA, the normal DECT band between 1.88 and 1.90 GHz cannot be used for transmission, but the generally accessible 2.4 GHZ ISM band (Industrial, Scientific, Medical) is available instead. Furthermore, changes would have to be carried out for matching to the national Standards, such as the American Standard xe2x80x9cFCC PART 15xe2x80x9d(Federal Communications Commission). This American Standard describes the transmission method, transmission powers and available bandwidth allowed for the radio interface.
In the DECT Standard, in addition to the 320 information bits mentioned above, each time slot also contains another 104 bits required for signal transmission, as well as 56 bits in guard field, so that each time slot contains a total of 480 bits. This results in a data rate of (24xc3x97480 bits)/10 ms=)1.152 Mbits/s. A data rate at this level is unusable in the American ISM band, since the bandwidth required per usable channel would be too large.
The publication, IEEE Personal Communications 1 (1994), New York, No. 2, 2nd Quarter 1994, pages 36 through 35, P. D. Rasky et al., xe2x80x9cSlow Frequency-Hop TDMA/CDMA for Macrocellular Personal Communicationsxe2x80x9d, discloses a method for digital radio transmission of data between a fixed station and at least one mobile station on one of a plurality of carrier frequencies, in which the data are transmitted in time slots in a time-division multiplex method and a time-duplex method. The change from one carrier frequency to another carrier frequency requires a predetermined time span on the order of magnitude of a time slot, and the data are transmitted in at least two respectively active time slots that follow one another, which are followed by a time slot in which no data are transmitted.
A The present invention thus has the object of providing a method and an arrangement for digital radio transmission of data which allow effective use of the bandwidth of a TDMA system. The method and the arrangement allows particularly , cost-effective use of the slow hopping RF modules.
According to the invention, a method is thus provided for digital radio transmission of data between a fixed station and at least one mobile station at one of a plurality of carrier frequencies. The data are transmitted in time slots (slots) using a time-division multiplex method (TDMA) and a time-duplex method (TDD). The change from one carrier frequency to another carrier frequency requires a time period corresponding to at least one time slot. The cost-effective slow hopping RF modules can thus be used. In order to allow effective utilization of the given bandwidth, the data are transmitted in at least two successive active time slots, which are followed by a time slot in which no data are transmitted, whereby the transmission from the fixed station to at least one mobile station as well as the transmission from at least one mobile station to the fixed station respectively occurs in six time slots.
In particular, the data can in each case be transmitted in two successive active time slots.
The transmission in the at least two successive active time slots can be carried out at the same carrier frequency. This allows the use of cost-effective slow hopping RF modules which cannot change the carrier frequencies from one time slot to the next.
The transmission may take place, for example, in a 2.4 GHz band.
Furthermore, an arrangement for digital radio transmission of data is provided according to the invention. This arrangement has a fixed station and at least one mobile station, between which the data can be transmitted in time slots using the time-division multiplex method (TDMA) and time-duplex method as well as at a plurality of carrier frequencies using the frequency-division multiplex method (FDMA). The fixed station at least one mobile station in this case each have an RF module which chooses the carrier frequency for transmission during one of the time slots. The RF modules require at least one time period corresponding to one time slot in order to change from one carrier frequency to another carrier frequency (slow hopping RF module). After two successive active time slots in which data are transmitted, an inactive time slot is provided, in which no data are transmitted, whereby the transmission from the fixed station to at least one mobile station as well as the transmission from at least one mobile station to the fixed station respectively occurs in six time slots.
The carrier frequency in the at least two successive active time slots may be the same.
The carrier frequencies may be in a 2.4 GHz band.
As so-called slow hopping RF modules, the carrier frequencies may be changed during the inactive time slot.